This invention relates to a monitoring system for monitoring digital signals flowing through a subscriber line.
It is necessary to monitor speech signals flowing through a subscriber line connected to an exchange for supervising the subscriber line. To this end, a proposal is made about various testing systems, namely, monitoring systems. For example, a monitoring system is disclosed in U.S. Pat. No. 4,447,679 issued to Takafumi Kojima et al and assigned to Hitachi, Ltd., et al. Specifically, the monitoring system comprises an exchange, a subscriber line coupled to the exchange, and a subscriber line test equipment coupled to the exchange through a subscriber line test link. The exchange comprises a subscriber circuit connected to the subscriber line, a test network, a compensation circuit, a control unit, and a time division network. The subscriber line test link is connected to the subscriber circuit through the test network and the compensation circuit.
Analog speech signals on the subscriber line are converted to digital speech signals in the time division network. The digital speech signals are sent out of the exchange through the time division network. In the monitoring system, the analog speech signals are monitored by the subscriber line test equipment and evaluated by an operator by listening to the analog speech signals.
On bidirectional communication, digital speech signals flow in both of uwpard and downward directions through a two-wire subscriber line. The digital speech signals which flow in the upward direction from the subscriber circuit to the exchange will be called upward digital speech signals while the digital speech signals flowing in the downward direction will be called downward digital speech signals. Under the circumstances, the upward and the downward digital speech signals coexist in the test link. Consequently, the upward and the downward digital speech signals interfere in the subscriber line test equipment. It is therefore impossible to monitor digital speech signals flowing through the two-wire subscriber line.